Courtney Hunt's low-budget blue-collar thriller, Frozen River, is one of the most impressive feature debuts of the past several years. It's set in and around the physically beautiful but socially deprived Mohawk reservation that's located partly in upstate New York and partly in southern Quebec, on either side of the US-Canadian border marked by the St Lawrence river. It's an area that is largely unfamiliar to the public at large, though back in the 1950s The Observer was instrumental in briefly bringing it to worldwide attention.

In 1957 the novelist and critic John Wain was commissioned by this paper to interview America's greatest man of letters, Edmund Wilson, at his upstate New York home and happened to ask him about the condition of the local Indians. It wasn't a matter to which Wilson had given any thought, and he gave Wain a vague reply. A few weeks later Wilson read a news report about Mohawk Indians moving in on some nearby ground they claimed as theirs. Crediting Wain for exciting his interest, he embarked upon a series of articles for the New Yorker that in 1960 became Apologies to the Iroquois, a seminal work in drawing attention to the plight and struggles of Native Americans.

Frozen River brings together two women, one white, the other a Mohawk Indian (one of the six tribes that make up the Iroquois nation), in the days before Christmas, and initially we're not invited to like either. We first see the bedraggled Ray Eddy (Melissa Leo) with a cigarette between her filthy fingernails, preparing for another desperate day coping with her two sons, one six, the other 15, in her rundown small-town home. Her husband, a chronic gambler, has just left, taking with him the next payment on a prefab dream house she's expecting to have delivered. Her job at the local Yankee Dollar convenience store is under threat (though she pretends a promotion is imminent), and her life is as bleak as the snow-covered landscape traversed by muddy roads. Searching for her husband, she sees his car being driven away by an obese young Indian woman. Ray follows her to a battered caravan on the Mohawk reservation and discovers her quarry to be the surly Lila Littlewolf (Misty Upham), who claims the car was abandoned by a man she saw catching a bus south with a young woman. The angry Ray pulls a gun on Lila, but after trying in vain to tow the car away, she agrees to Lila's proposition to sell it to one of the Reservation's smugglers.

Gradually we discover that both women are decent mothers struggling in desperate situations. Lila has also been deserted by her husband and has trouble holding down a job and supporting her baby who is being cared for by her mother-in-law. Ray will do anything to feed and house her kids and keep them in school. The two women are first mutually suspicious, then cautiously trusting partners in crime working as people-smugglers, driving Asian aliens hidden in the car boot across the frozen and unpatrolled St Lawrence into America. Within the reservation (where there's catastrophic unemployment), Ray is unwelcome, but Lila is pretty well invulnerable. In the States, with Ray as driver, the suspicions of the state troopers that Lila would arouse are allayed.

The pair, whom we come to care deeply about, are living on thin ice, and the frozen St Lawrence signifies both the opportunity to make quick money and the risk of disaster. The trafficking points to the terrible irony of two Americans living hand-to-mouth while facilitating the passage of foreigners who pay everything they've got to snakehead gangs to start a new life in the States. The excitement and suspense steadily escalate as things start to go wrong when a Pakistani couple hides a baby in their luggage. Then matters get even worse on that inevitable "last job": a gunfight breaks out in Canada, and a police pursuit ensues. There are some problematic moments in the plot, but the resolution is satisfying and achieved without rhetoric or sentimentality.

A lawyer turned filmmaker, Courtney Hunt experienced some of the same privations as Ray's boys, and interestingly cites Scorsese's Alice Doesn't Live Here Any More as a film that influenced her as a child. She brings a sharp eye to the world of Ray and Lila and is rewarded by excellent, wholly uningratiating performances from Leo, who was rightly nominated for an Oscar (and in my view should have won) and Upham. They work beautifully together. There is also striking support from Charles McDermott as Ray's troubled, resourceful teenage son, and from Michael O'Keefe as a sympathetic state trooper. A credit to the independent sector of the American cinema, Frozen River cost far less than the first two minutes of the new Harry Potter picture and is an hour shorter.